FLOOD00281

• County: Larimer
• Community: Greeley
• Title: Design Criteria and Construction Specifications volume 2 of 5 Storm Drainage Criteria
• Date: 10/1/1997
• Prepared For: Greeley
• Prepared By: Department of Public Works
• Floodplain - Doc Type: Educational/Technical/Reference Information

The investigations necessary to assure that the natural channels
will be adequate are different for every waterway. The engineer
must prepare cross sections of the channe!, define the water
surface profile for the i.nitial and major desiqn floexi, investigate
the bed and bank material to determine erosion t,endencies, and
study the stability of the channel bed and bank under future
conditions of flow. Super critical flow does 1101: normally occur in
natural channels, but calculations must be made to assure that the
results do not reflect super critical flow. If super critical flow
is present, drop structures or other appropria1:e energy dissipation
structures must be provided.

5.2.2

Grass Lined Channels

Grass lined channels are the most desirable of .the artificial
channels. The grass Idll stabilize the body of the channel,
consolidate the soil mass of the bed, check t:he E~rosion on the
channel surface, and control the movement of soil particles along
the channel bottom. The channel storage, the Imler velocities, and
the greenbelt multiple-.use benefits obtained l:reate significant
advantages over other artificial channels.

The presence of grass in channels creates turbulence which results
in loss of energy and increased flow retardancle. Therefore, the
designer must give full ,:onsideration to sediment deposition and to
scour, as well as hydraulics. Unless existing development within
the City restricts the availability of right-of-way, only channels
lined with grass will be considered acceptable for major
drainageways. Grass lined channels may require an acceptable
trickle channel, as defined in Section 5.4.2.

For the purpose of these Criteria, sandy soils are defined as non-
cohesive sands classified as SW, SP or 8M in accordance with the
Unified Soil Classificat:ion System.

5.2.3

Concrete Lined Channels

Concrete lined channels for major drainageways will be permitted
only where right-of-way restrictions within ,existing development
prohibit grass lined channels. The lining must be designed to
withstand the various forces and actions which tend to overtop the
bank, deteriorate the lining, erode the soil blenea.th the lining,
and erode unlined areas, especially for the super critical flow
conditions.

If the project constraints suggest the use o:E a. concrete channel
for a major drainageway, such use shall bE! allowed only upon
approval by the City. The applicant shall present the
justification and desi''In to the City for consideration of a
variance from these crit.eria.

A Design
channel.
city.

Report is required for approval of a concrete lined
The contents of such report shall be determined by the

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